You can but you will need to manually convert your call history (if you need it). After the upgrade to v15 locate and run CHconverter.exe from C:\Program Files\3CX Phone System\Instance1\Bin.
it will ask you for the path of the v12.5 backup file used in the upgrade, type in the location of the v12.5 backup.zip file and press enter. Call history will be pulled into v15 from the 12.5 backup.